The OP suggested that it's a bad option for phones. But Amazon offers something else for higher-end items that worked great for me,"Amazon Trade-in" (http://www.amazon.com/Trade-In/).

You start by finding your product in the store, and they give you prices for different condition levels. You pick a level, checkout, ship your items for free, and await receipt and review. If accepted as the condition you picked, you get an Amazon gift card for the amount. They might even upgrade your items to a higher condition.

If not accepted, your items are returned, free of charge. The only risk is the waste of time.

A month ago, I traded in two nearly-3-year-old iPhone 4's. I listed them as "Good". Both were accepted and one was upgraded to "Like New" for $20 more. I got $380 total, which I was extremely happy with.